Most of the qemuargv2xml tests are parsing old style qemu command
lines (with -disk, -serial, etc), and it gets its input from
qemuxml2argv output.

But since we've raise the minimum supported qemu version to 0.12.0,
which supports -device, once that changes propagates through libvirt
the vast majority of qemuxml2argv output is _not_ going to be using
old style qemu options.

In preparation for this, switch qemuargv2xml to use its own copies
of input and output, so it's not tied to qemuxml2argv results.

This is just a straight copy of the current tests.